(a) Field of the Invention
This invention relates to incremental distance measuring apparatus, particularly a scale casing or housing and a spar for holding and positioning the casing.
(b) History of the Prior Art
In high precision incremental distance measuring systems, such as illustrated in U.S. Pat. Nos. Re. 29,281, Re. 29,282 and 4,031,595an accurately ruled glass scale is supported along its length by a casing, typically an aluminum extrusion. Such glass scales have, as described in U.S. Pat. No. 4,031,595, 500 or more equal lines and spaces per inch. The instruments which incorporate such scales are, typically, accurate to from 0.0005 inches to 0.00005 inches.
In systems such as described above, it is important to insure the straightness of the glass scale. If straightness is not maintained, the distance along the surface of such a scale will vary, which will affect the accuracy of the measuring system. To minimize bending, a variety of apparatus and procedures are employed to secure the casing to, for instance, machine tools. In the system illustrated in U.S. Pat. No. 4,031,595, the casing is mounted to the table of a machine tool by a plurality of bolts spaced long the upstanding portion of the casing. In the system illustrated in U.S. Pat. No. Re. 29,282, the casing is secured to the machine tool via a dovetail. In other systems, the casing is fastened to the machine tool at its ends, or supported by one or more brackets, or a combination of both. Cement is also used to help secure the casing to a machine tool. However, as the length of scales and, hence, supporting casings has increased, and as product miniaturization has resulted in the reduction of the cross-sectional area of the casing, the problem of maintaining the straightness of the scale has increased.
Brackets used in the prior art for holding the casings have several disadvantages. For long casing lengths, they often do not provide enough support to maintain the straightness of the casing and, hence, the glass scale. In addition, brackets are often bulky, which either limits or prohibits their use in certain mounting situations. Additionally, brackets do not provide for protection of the sides and top of the casing.